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Spark plug condition

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Spark plug condition

Post by mm5aho on Mon Apr 01 2013, 18:49

After getting the last parts of the Lister D assembled, I tried to fire it up. It doesn't go. There were a few coughs to start with, but I think the spark is weak. The fule seems OK, and in fact after enough turning the plug gets wet, so fuel is getting in there.

The plug is very old, and in poor condition. I'll change this for a new one I think, but meantime I checked the magneto. I had taken this compeletly apart, cleaned and reassembled. It seemed to have the right symptoms. Definite resistance to turning as the magnets engage the coil (magnetically, not physically). The points look OK, and the timing of the points opening aligns with the Z on flywheel.
When turning over with the spark plug out and body earthed, the spark is intermittent. I suspect the insulation is cracked, but can't see a crack.

Any hints on how to test a spark plug?
Any ideas what else to check for?


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Re: Spark plug condition

Post by matt86 on Mon Apr 01 2013, 19:59

just put a new plug on the end and try that as there only a few £'s earth the body out and if a intermitant spark could be possible coil damage at the worst .

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Re: Spark plug condition

Post by blackvanman on Mon Apr 01 2013, 21:26

I test plugs by doing a 1000v insulation test, and grit blast to clean them Smile

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Re: Spark plug condition

Post by stationary stu on Tue Apr 02 2013, 10:53

Sorry but why worry about checking the plug? it's not as if you can repair it, or can you!!!! I know the old plugs came apart for cleaning but don't know if you can get the parts to repair one of those.
Have you tried closing the plug gap down to about 15 - 18thou see if that helps.

Stu.

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Re: Spark plug condition

Post by jay on Tue Apr 02 2013, 14:35

easy way of testing the mag is spin it with a drill with a socket in the chuck.have the lead a mm from the mag body if its sparking all the time its usually ok..but make sure the points are in good condition and clean with no oil on the face of the points.
ngk A6 or 7 will be ok for lister D use,but modern plugs dont like being flooded with petrol or brushed with a wire brush as it remove the insulation.

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Re: Spark plug condition

Post by mm5aho on Tue Apr 02 2013, 15:34

My logic in want to check the pug is as follows:

1. Engine won't start. Known to be mechanically OK as only just reassembled. Compression OK.
2. Not firing is one of two things.. fuel problem or spark problem.
3. Test for fuel. Carb float bowl full? Yes, fuel to carb? yes. Does plug get wet with fuel after serious cranking? yes. So probably nota fuel issue.
4 Assuming its a spark issue, then test for spark; is there any? If so is at the right timing? and right strength?
5. Check spark timing. Retime the magneto to the flywheel: OK. Now test with spark plug out of block, earthing the body of plug. Is there a solid blueish spark? or weak yellowish? Its blue. Is it regular? No, its intermittent. (and this isn't a "hit and miss"). So, could be magneto coil or plug lead breakdown or the plug itself.

So wanting to eliminate some of the possibilities, can I test the plug in any way?

And that's where I got to. I have a new plug ordered, might arrive tomorrow, so will try new plug to eliminate that possibility. If there's still a problem might look closer at magneto, and plug lead and its connections.

It will go yet! Never admit defeat!

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Re: Spark plug condition

Post by stationary stu on Wed Apr 03 2013, 12:20

I have a few spare plugs that's new that I picked up cheap just so I can test the spark.

I also have one of these but it doesn't fit that many engines but with a bit of fiddling and a few crocodile clips I can get a result.

http://www.ebay.co.uk/itm/IGNITION-SPARK-TESTER-270-MM-CONNECTS-BETWEEN-SPARK-PLUG-IGN-LEAD-/400448473998?pt=UK_Hand_Tools_Equipment&hash=item5d3c96cb8e

Stu.

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Re: Spark plug condition

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